1. Field of the Invention
The present invention relates to a process for converting crude pigments such as crude quinacridone to an aggregated low crystallinity form followed by conversion to a pigmentary form by milling in the presence of an aqueous base which may contain a salt or an aqueous basic salt and a small amount of an organic liquid at least some of which is present as a distinct second phase.
It is well known in the art that quinacridones as synthesized, known as crude quinacridones, are generally unsuitable for use as pigments and must be further processed to develop the requisite pigmentary properties such as particle size, particle shape, crystalline structure, tinctorial strength, phase, etc. The most commonly used processes for converting crude quinacridones to pigmentary form involve milling the crude quinacridone with large quantities of inorganic salt and then extracting the resulting mill powder. While the salt grinding process produces satisfactory pigment for some applications, it suffers from two important disadvantages. First it involves the use of large quantities of salt which severely limits the amount of pigment which can be charged to the mill and which has to be disposed of and second the resulting pigment particles often are acicular which creates rheological problems in final applications such as high solids acrylic paints for automotive use.
2. Prior Art
Various attempts to eliminate the use of salt in milling crude quinacridones have been made. For instance, in U.S. Pat. No. 2,857,400 crude pigment is premilled in a ball mill, then homogenized in acetone and recovered from the acetone. In U.S. Pat. No. 3,017,414 crude pigment is premilled in a ball mill, then treated with a water-insoluble organic liquid such as chlorobenzene in water emulsion, and recovered from the emulsion.
Another method for preparing pigmentary quinacridone from crude quinacridone, is described in U.S. Pat. No. 3,287,147 wherein the crude quinacridone is either acid pasted or ball milled to provide a product which is formed into a neutral aqueous paste which is then heated at 150.degree. to 300.degree. C. under pressure. Special equipment is required for heating at elevated temperatures and pressure.
U.S. Pat. No. 4,024,148 discloses preparation of a novel crystalline form of quinacridone by conditioning precipitated quinacridone in an aqueous slurry thereof in the presence of a large quantity of a water-insoluble liquid having at least one hydroxyl group and a surfactant. The surfactant is retained on the surface of the pigment.
In still another method (U.S. Pat. No. 4,094,699) which dispenses with the use of organic solvents premilled quinacridone is ripened in an aqueous base in the presence of both cationic and nonionic surfactants. Some of the surfactants are retained on the surface of the pigment and can cause compatibility problems in some end-use systems.